Investments and Fair Value Measurements - Amortized Cost to Fair Value Reconciliation (Details) - USD ($) $ in Thousands	Jul. 31, 2020	Jan. 31, 2020
Debt Securities, Available-for-sale
Amortized Cost	$ 586,226	$ 1,003,896
Unrealized Gains	2,547	1,273
Unrealized Losses	0	(23)
Fair Value	588,773	1,005,146
Due within one year	588,773
Cash and Cash Equivalents \| US Treasury Securities
Debt Securities, Available-for-sale
Amortized Cost		8,035
Unrealized Gains		0
Unrealized Losses		0
Fair Value		8,035
Investments, Current \| US Treasury Securities
Debt Securities, Available-for-sale
Amortized Cost	530,843	866,578
Unrealized Gains	2,077	590
Unrealized Losses	0	(23)
Fair Value	532,920	867,145
Investments, Current \| Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost	55,383	103,848
Unrealized Gains	470	521
Unrealized Losses	0	0
Fair Value	$ 55,853	104,369
Investments, Current \| Commercial Paper
Debt Securities, Available-for-sale
Amortized Cost		4,991
Unrealized Gains		3
Unrealized Losses		0
Fair Value		4,994
Investments, Non-current \| Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost		20,444
Unrealized Gains		159
Unrealized Losses		0
Fair Value		$ 20,603

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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